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Investments - Internally Managed Investment Portfolio (Details) - USD ($) $ in Millions	Jun. 30, 2023	Dec. 31, 2022
Debt and Equity Securities, FV-NI [Line Items]
Estimated fair value	$ 6,488	$ 7,119
Fixed-maturity securities, trading	340	303
Short-term investments, at fair value	1,650	810
Other invested assets	146	133
Total investments	8,624	8,365
Externally managed
Debt and Equity Securities, FV-NI [Line Items]
Estimated fair value	5,551	5,824
Externally managed | Assured IM
Debt and Equity Securities, FV-NI [Line Items]
Estimated fair value	292	305
Loss Mitigation Securities
Debt and Equity Securities, FV-NI [Line Items]
Estimated fair value	509	548
Puerto Rico, New Recovery Bonds
Debt and Equity Securities, FV-NI [Line Items]
Estimated fair value	36	358
Other
Debt and Equity Securities, FV-NI [Line Items]
Estimated fair value	392	389
Other | Assured IM
Debt and Equity Securities, FV-NI [Line Items]
Estimated fair value	$ 221	$ 232
BIG
Debt and Equity Securities, FV-NI [Line Items]
Fixed-maturity investments, non-investment grade, percent	7.50%	7.40%
Not Internally Rated
Debt and Equity Securities, FV-NI [Line Items]
Fixed-maturity investments, non-investment grade, percent	1.70%	5.90%
Equity method investments
Debt and Equity Securities, FV-NI [Line Items]
Other invested assets	$ 130	$ 123
Other
Debt and Equity Securities, FV-NI [Line Items]
Other invested assets	$ 16	$ 10

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Amount excluding accrued interest, of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

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-Topic 326
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-Name Accounting Standards Codification
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Sum of the carrying amounts as of the balance sheet date of all investments.

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